Simultaneous Multi-Spot Inspection And Imaging

ABSTRACT

A compact and versatile multi-spot inspection imaging system employs an objective for focusing an array of radiation beams to a surface and a second reflective or refractive objective having a large numerical aperture for collecting scattered radiation from the array of illuminated spots. The scattered radiation from each illuminated spot is focused to a corresponding optical fiber channel so that information about a scattering may be conveyed to a corresponding detector in a remote detector array for processing. For patterned surface inspection, a cross-shaped filter is rotated along with the surface to reduce the effects of diffraction by Manhattan geometry. A spatial filter in the shape of an annular aperture may also be employed to reduce scattering from patterns such as arrays on the surface. In another embodiment, different portions of the same objective may be used for focusing the illumination beams onto the surface and for collecting the scattered radiation from the illuminated spots simultaneously. In another embodiment, a one-dimensional array of illumination beams are directed at an oblique angle to the surface to illuminate a line of illuminated spots at an angle to the plane of incidence. Radiation scattered from the spots are collected along directions perpendicular to the line of spots or in a double dark field configuration.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/553,174,filed Oct. 26, 2006, which is a continuation of application Ser. No.10/418,352, filed Apr. 17, 2003, now U.S. Pat. No. 7,130,039; whichapplication is a continuation-in-part of application Ser. No.10/125,906, filed Apr. 18, 2002, now abandoned, and which also claimsthe benefit of Provisional Application No. 60/426,577, filed Nov. 15,2002. These applications are incorporated herein in their entirety byreference as if fully set forth herein.

BACKGROUND OF THE INVENTION

This invention relates in general to the inspection of surfaces todetect anomalies, and in particular, to an improved system thatilluminates the surface inspected at the plurality of spotssimultaneously for anomaly detection.

Conventional optical inspection methods employing scanning techniquestypically causes a single spot on the surface inspected to beilluminated where the spot is scanned over the entire surface foranomaly detection. For improved signal-to-noise ratio caused bybackground scattering, the size of the illuminated spot has beencontinually reduced. This means that the amount of time required for thespot to scan over the entire surface is increased which is undesirable.

One solution to the above dilemma is proposed in U.S. Pat. No.6,208,411, which is incorporated herein by reference in its entirety.This patent proposes a massively parallel inspection and imaging systemwhich illuminates the surface at a plurality of spots where scatteredlight from the spots are imaged onto corresponding detectors in adetector array.

While the system in U.S. Pat. No. 6,208,411 provides a major enhancementin the total inspection throughput, it may be further improved forenhanced performance in certain applications. It is, therefore,desirable to provide an improved multi-spot inspection and imagingsystem with enhanced characteristics.

SUMMARY OF THE INVENTION

While the system described in U.S. Pat. No. 6,208,411 provides a majorenhancement in the total inspection throughput, for some applications,it may be desirable for the system to be compact and have a smallerfootprint. In such event, it may be desirable for the focusing opticsfocusing multiple beams of radiation to an array of spots and theimaging optics imaging scattered radiation from the spots to an array ofreceivers or detectors to employ different objectives. In one embodimentof the invention, the objective used for imaging has a larger numericalaperture than the objective use for focusing. This enhances detectionsensitivity.

In another embodiment, radiation reflected from an array of illuminatedspots on the surface may be imaged onto a first array of receivers ordetectors in a bright field detection configuration and radiationscattered from the spots may be imaged onto a second array of receiversor detectors in a dark field detection configuration. The use of bothbright and dark field detection provides more information for anomalydetection.

In yet another embodiment, the multiple beams of radiation are focusedto an array of spots on the surface where the radiation comprises atleast one wavelength component in the ultraviolet (“UV”) or deepultraviolet range of wavelengths. Scattered or reflected radiation fromthe spots are imaged by means of optics that comprises a reflectiveobjective to reduce chromatic aberration.

In still another embodiment in a compact and modular approach, anoptical head for anomaly detection includes illumination optics focusingillumination beams of radiation to an array of spots on a surface andimaging optics that images scattered or reflected radiation from thespots onto an array of optical fibers. Each of the fibers containsscattered or reflected radiation from one of the spots. Such light maybe supplied to detectors outside the optical head for processing andanomaly detection. Instead of optical fibers coupled to detectors, othertypes of receivers may also be used.

In still another embodiment of the invention, in addition to the opticalhead described immediately above, a plurality of detectors generatesignals in response to radiation scattered or reflected by the surfaceor radiation from the fibers and rotational motion is caused between thesurface and the illumination beams so that the beams are scanned oversubstantially the entire area of the surface. Where the surfaceinspected is that of a semiconductor sample having multiple dicethereon, signals from the detectors from at least two dice or portionsthereof of the surface are stored as the beams are scanned over thesurface. Preferably, the scattered radiation from the two dice may becompared in a die-to-die comparison for anomaly detection.

The surface inspected sometimes has diffraction patterns thereon. Insuch event, scattered or reflected radiation from the array ofilluminated spots on the surface may be masked by diffraction from thepattern. Thus, in another embodiment of the invention, when relativerotational motion is caused between the surface inspected and theillumination beams, a spatial filter in or near the focal plane of theimaging objective having an aperture that is caused to movesubstantially in synchronism with the rotational motion to reducediffraction from the pattern that is passed to the array of receivers ordetectors. Alternatively, as rotational motion is caused between thesurface and the beams, a stationary filter in the shape of an annularaperture may be employed to shield the detectors from patterndiffraction. For some applications, both types of filters may be used atthe same time during inspection.

In still one more embodiment, beams of radiation are focused to an arrayof elongated spots on the surface at oblique angle(s) of incidence tothe surface where the centers of the spots are arranged along asubstantially straight line. Scattered radiation from the spots areimaged onto corresponding receivers or detectors in an array by means ofimaging optics with a focal plane that substantially contains all of thespots.

In yet another embodiment, the same optics is used for focusingillumination beams of radiation to an array of spots on a surface andfor imaging scattered radiation from the spots onto correspondingreceivers or detectors in an array. The optics has an aperture where theillumination beams are focused through a first portion of the apertureand the imaging occurs through a second portion of the aperture.Preferably, the second portion is larger than the first portion, whichenhances the sensitivity of detection.

It should be noted that any one or more features in the above-describedembodiments may be employed in combination with one or more features ofa different embodiment for enhanced performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a multi-spot dark-field/bright-fieldinspection and imaging system to illustrate an embodiment of theinvention.

FIG. 2 is a schematic view of a two-dimensional arrangement of multipleilluminated spots on the surface inspected to illustrate the embodimentof FIG. 1.

FIG. 3 is a schematic view of the multiple spots of FIG. 2 and theirscan paths across the surface inspected to illustrate the embodiment ofFIG. 1.

FIG. 4 is a schematic view illustrating the scan paths of two adjacentspots to illustrate the embodiment of FIG. 1.

FIG. 5 is a schematic view of a spatial filter in the collection path ofthe embodiment of FIG. 1 to further illustrate the embodiment.

FIG. 6 is a schematic view of an annular-shaped spatial filter that maybe used in the in the collection path of the embodiment of FIG. 1 toillustrate the invention.

FIG. 7 is a schematic view of an annular-shaped illuminated region ofthe surface inspected containing two dice to illustrate one aspect ofthe invention in the embodiment of FIG. 1.

FIG. 8 is a schematic view of an optical inspection and imaging systemto illustrate another embodiment of the invention.

FIG. 9 is a top schematic view of an optical inspection and imagingsystem to illustrate yet another embodiment of the invention.

FIG. 10 is a schematic side view of an optical inspection and imagingsystem to illustrate still another embodiment of the invention in asingle dark field configuration.

For simplicity in description, identical components are labeled by thesame numerals in this application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The costs associated with dark-field pattern inspection has increasedsteadily with enhanced performance. As semiconductor fabricationapproaches finer design rule and resolution, the complexity ofinspection tasks has increased dramatically, which, in turn, increasesthe complexity and costs of the optical front end of the inspection tooland of detection electronics. Furthermore, the variety of situationscalling for optical inspection means that a versatile optical inspectiontool is preferably compact, rugged and has a small foot print so that itis less sensitive to vibrations, and integratable with semiconductorprocessing equipment. Preferably, the system can be used for inspectingsurfaces with diffracting patterns thereon such as patterned wafers, aswell as surfaces without such patterns such as unpatterned semiconductorwafers. The embodiments of this invention also enable faster and moresensitive inspection to be performed at a reasonable cost.

The elements of the optical front-end design (such as those in anoptical head) of the proposed system 20 are shown in FIG. 1. Theradiation from a laser 22 is first split into an array 24 of beams,preferably a two-dimensional array, by the action of a diffractiveoptical element 26 a on substrate 26. These beams are simultaneouslyfocused onto the surface of a sample such as a semiconductor wafer 28,placed on a spinning stage, preferably a precision spinning stage, by alens such as a simple doublet lens 30. Preferably lens 30 has anumerical aperture of not more than 0.8. The radiation scattered offeach spot is collected by a reflecting objective 32, and imaged by anobjective 38 onto a corresponding fiber in an M by N array 34 of opticalfibers arranged to correspond to the distribution of the spots on thewafer. These fibers carry the radiation to an array 36 of avalanchephotodiodes (APD), amplifiers, and digitizers. Other types of detectorsare possible and may be used as described below. Alternatively, insteadof imaging the scattered radiation collected from each spot on the waferto an optical fiber, it may be imaged onto a detector in a detectorarray. In the embodiment of FIG. 1, the illumination beams 24 aredirected towards the wafer surface in directions that are substantiallynormal to the surface of the wafer. Preferably the beams illuminate onthe wafer surface spots that are substantially circular in shape.

Preferably, beams 24 reach sample 28 in directions substantially normalto the sample surface, and the collection or imaging optics comprisingobjectives 32, 38 are rotationally symmetric about a line 64 normal tothe sample surface. As noted above, preferably the beams 24 illuminateon the wafer surface spots that are substantially circular in shape. Inthis manner, the illumination beams and the collection optics arerotationally symmetric about line 64. Then there is no need to keeptrack of the orientations of the illumination and collection opticsrelative to any diffraction or scattering patterns from the samplesurface. Relative motion is caused between beams 24 and the samplesurface so that preferably the beams trace or scan spiral paths on thesample surface, in a manner described below.

The orientation of the spots 42 illuminated by the array 24 of beams isslightly rotated with respect to the tangential direction x of the waferas the wafer is rotated as shown in FIG. 2. The spots along a givencolumn “paint” adjacent paths as shown in FIG. 3. In a xy coordinatesystem 41, the thick arrow 46 illustrates the y direction of the imageobtained. The separation between the adjacent spots is chosen so as tosatisfy a desired sampling level (e.g. 3×3 or 4×4 samples per spotwidth). The detectors are sampled at uniform intervals in time at a rateof typically 3 or 4 samples per point spread function (PSF). This isillustrated in FIG. 4, which shows the paths of two adjacent spots, suchas spots 42 a and 42 b in FIG. 2 travelling along paths 44 a and 44 brespectively. The two paths may be offset by a separation dsubstantially equal to one-third or one-quarter of the spot size toachieve the 3×3 or 4×4 samples per spot width, so that the spots 42 aand 42 b would overlap by two-thirds or three-quarters of the spot size.Thus, a one-dimensional scan of the wafer produces a two dimensionalimage, as illustrated in FIG. 3.

The optical components in the design are quite simple. The multi-beamsplitter 26 a may be one similar to the grating element used for asimilar purpose in U.S. Pat. No. 6,208,411, where the element is aspecially designed diffractive optical element. In choosing the totalnumber of spots 42, it is desirable to pay attention to the total systemcomplexity including, in particular, costs associated with theelectronics. It has been determined that a total number of 128 channelsis a reasonable compromise. This is achieved through a 16×8 array ofspots. Other combinations are also possible. In some applications, theuse of an odd number of spots such as 17×7 may be advantageous. Theangular orientation of the spots with respect to the tangentialdirection of the wafer is chosen such that the spots in a columntraverse the space between any two adjacent horizontally positionedspots (FIG. 2), resulting in a complete coverage of a swath of thewafer. In one embodiment, the separation between the spots is chosensuch that 4 samples per point spread function (PSF) are attained in eachdirection. This is a slightly denser sampling than in the case of theAIT™ system available from KLA-Tencor Corporation of San Jose, Calif.The denser sampling reduces processing time, because smallerinterpolation kernels are allowed for the same level of residualinterpolator truncation error. The fact that the scan is spiral alsofavors a denser sampling, since the interpolation is inherently morecomplex than for a rectilinear scanner.

The intensity profiles of the spots are Gaussian shaped with a 1/e²intensity width of 5 microns, for example. At a sampling level of 4×4per spot width, the total width (i.e. swath width) of the tracks of the128 (for a 16 by 8 array) spots is about 160 microns. In this context, atrack is the locus of a spot as the sample is scanned. The maximumamount of the beam fan out in at the focusing lens 30 is so small thatonly a simple doublet suffices for focusing. Other types of lenses mayalso be used instead.

The dark-field collector in this design is a reflecting objective 32,placed directly above the illuminated field. While a 0.5 numericalaperture (NA) lens may be used for objective 32, lenses of other NA arepossible. The reflecting lens performs two tasks: i) it collects theradiation scattered off each spot, and ii) it images the spots onto acorresponding array of fibers. The separation of the spots on the waferis such that they can be considered as completely independent, withoutinter-spot interferences.

The radiation provided by laser 22 may contain one wavelength componentor more than one wavelength component. Such radiation may include awavelength component in the ultraviolet range, deep ultraviolet range,visible or infrared range, or wavelength components in more than one ofthe four different wavelength ranges. The laser or other radiationsource 22 may operate in the visible, infrared, ultraviolet or deepultraviolet range or ranges. An attraction of using a reflectingobjective such as mirror 32 is that it functions well over a large rangeof wavelengths. For some applications, a refractive objective may alsobe used instead of a reflecting one for collecting and imaging scatteredradiation from the wafer 28 to the fiber array 34.

Laser 22 may emit radiation of substantially a single wavelength in thesystem. Alternatively, laser 22 may emit radiation of a plurality ofwavelengths, although radiation of only one of the plurality ofwavelengths is used at any one time for inspection. The diffractingelement 26 a is preferably placed at the back focal plane of lens 30, sothat the beams 24 are focused to the surface of the wafer 28, where theaxes of beams 24 are substantially parallel to one another andperpendicular to the wafer surface, satisfying the condition oftelecentricity.

Where radiation of a different wavelength is employed in scanning thesample surface (such as where laser 22 has more than one possiblewavelength), the spot separation may change if the same element 26 a isused to diffract the laser beam, since diffraction by element 26 a iswavelength dependent. In such event, a different diffraction elementsuch as element 26 b may be used to compensate for the change inwavelength so that the spot separation remains substantially the same.Beam forming optics (not shown) may be used to change the width of thebeam from the laser in order to maintain the same spot size as before,so that the collection optics in the system need not be changed. Thisswitching between diffracting elements 26 a and 26 b can be accomplishedreadily by moving substrate along direction 27 using means such as amotor (not shown in FIG. 1). Since phase changes are not of interest andare not detected, there is no requirement to align precisely the element26 b with respect to the beam. Obviously more than two diffractionelements may be formed on the same substrate 26 in the event the laserbeam contains more than two wavelength components.

Instead of changing the diffracting element when radiation of differentwavelength is used, the same spot separation and spot size as before canbe achieved by altering the focal length of the focusing lens 30 inFIG. 1. Then the collection optics also need not be changed. However,since the diffracting element is preferably placed at the back focalplane of lens 30, once the focal length of the lens is altered, theelement needs to be moved to a new location which is again at the backfocal plane of the lens 30. Where it is desirable to change the spotseparation and spot size, one can alter the wavelength of the radiationused to inspect the wafer without changing the illumination optics.However, the imaging optics may then need to be altered by changing themagnification of the lens 38 so that lens 38 will still focus thecollected radiation from the spots and image onto the fibers. To obtaina different spot separation and spot size without changing thewavelength of the radiation used to inspect the wafer, one can alter thefocal length of lens 30, or alter the diffracting element and beamforming optics. Such and other variations are within the scope of theinvention.

Where laser 22 emits more than one wavelength component, appropriatewavelength selection optical elements such as filters or beam splitters(not shown) may be employed in the path of the beam from laser 22 toselect the component of the desired wavelength, so that radiationsubstantially at only one selected wavelength is supplied to element 26a or 26 b at any one time. In such event, laser 22 and the wavelengthselection optical elements form an optical source that suppliesradiation of a selectable wavelength from a plurality of wavelengths.Obviously, other types of optical source that supplies radiation of aselectable wavelength may be used instead. Thus, alternatively, wherelaser 22 emits monochromatic radiation, a different laser emittingradiation of a wavelength different from that emitted by laser 22 may beemployed to replace laser 22. As another alternative, separatemonochromatic or polychromatic lasers may be combined by means such asdichroic filters to provide radiation of selectable wavelength. Such andother variations are within the scope of the invention.

In a system for enhanced detection sensitivity, it is desirable for thecollection optics such as objective 32 to have a large numericalaperture (NA) whereas for the illumination optics such as lens 30, a lowNA will be sufficient. System 20 illustrated in FIG. 1 shows aparticularly compact design where the illumination optics and collectionoptics employ different objectives, that is objectives 30 and 32, wherethe collection objective 32 has a larger NA than that of theillumination objective 30. By using low NA illumination optics, it ispossible for both the illumination optics and collection optics to fitwithin the space close to the wafer 28 in a particularly compact designof the optical head, as shown in FIG. 1.

Thus, according to another aspect of the invention, the optical head inthe embodiment of FIG. 1 is compact and has a particularly smallfootprint. Thus, optical head 60 within the dotted line box includes alaser 22, diffractive elements 26 a and 26 b, lens 30, collectionobjective 32, and the array of optical fibers 34. In a slightly modifiedembodiment than that shown in FIG. 1, laser 22 may also be locatedoutside the optical head 60 and may be placed so that its output laserbeam is directed to the diffractive element 26 a in the optical head 60,possibly by means of an optical fiber link. Such and other variationsare within the scope of the invention.

The combination of the collection objective 32 and objective 38 focusesradiation scattered from each illuminated spot 42 on the surface ofwafer 28 to a corresponding optical fiber in the optical fiber array 34.Scattered or reflected radiation from each spot is then carried by itscorresponding fiber to a detector which may be an avalanche photodiode,a pin photodiode, a photomultiplier, or other individual detector. Thedetector may be part of an array 36 of detectors. By using an opticalfiber array 34, the detectors 36 do not need to be included in theoptical head 60 and can be located at a distance from the optical head,thereby further reducing the size of the optical head. Alternatively,for applications where spatial considerations are not as important, theoptical fiber array 34 may be omitted and the scattered radiation fromeach spot may be focused directly by objective 32 to a correspondingdetector in the detector array 36 within the optical head. Such andother variations are within the scope of the invention.

In the above description, element 26 a diffracts the laser beam fromlaser 22 into a two-dimensional array of beams 24. Instead ofdiffracting the laser beam into a two-dimensional array of beams,element 26 a may instead diffract the beam into a one-dimensional arrayof beams to illuminate a one-dimensional array of illuminated spots onthe surface of the wafer 28. Such one-dimensional array of illuminatedspots may, for example, comprise the five illuminated spots appearing asthe right most column 42′ in FIG. 2. Another example of suchone-dimensional array of beams and spots is illustrated in FIG. 9. Thepaths of illuminated spots in column 42′ may also overlap as indicatedin FIG. 4. Such and other variations are within the scope of theinvention.

Bright Field Detection

Bright field detection is where specularly reflected radiation isdetected, such as that described in S. Stokowski and M. Vaez-Iravani,“Wafer Inspection Technology Challenges for ULSI Technology”,Proceedings of conference on Characterization and Metrology for ULSITechnology, Edited by D. G. Seiler, A. C. Diebold, W. M. Bullis, T. J.Shaffner, R. McDonald, and E. J. Walters, American Institute of Physics,PP. 405-415 (1998).

In the embodiment of FIG. 1, the array of illumination beams 24 arefocused by lens 30 to a mirror 62 which reflects the beams towards wafer28. Mirror 62 also acts as an aperture stop to reduce or preventspecular reflection of the beams from the surface 28 from reaching theoptical fiber array 34, so that the collection mirror 32 collects onlyradiation scattered by the spots along collection paths that are awayfrom the specular reflection direction in a dark field imaging system.Dark field systems are those where the radiation collected and detectedis that scattered by the sample and collected along collection pathsthat are away from the specular reflection direction from the samplesurface of the illumination beams. Dark field systems are explained inmore detail in the above-referenced article by S. Stokowski and M.Vaez-Iravani.

FIG. 1 also shows the reflected path into the “bright-field” channels70, which may comprise an optical fiber array similar to array 34. Thebeams 24 from element 26 a are first reflected by a beam splitter 66towards lens 30 and mirror 62. Part of the radiation specularlyreflected by the wafer surface is again reflected by mirror 62,collimated by lens 30 and passes through the beam splitter 66 towardsbright-field channels 70 and then to an array of detectors (not shown).As in the case of dark field detection, the radiation reflected fromeach spot is imaged by lens 30 onto a corresponding channel in channels70 and then to a corresponding detector. Also as in the dark fieldsystem, the detector array in the bright field system need not beincluded in optical head 60 for compactness. Where space is not as mucha concern, channels 70 may be replaced by an array of detectors so thatlens 30 and simple optics (not shown) located downstream from lens 30 inthe same optical path image radiation reflected by each spot directly tothe corresponding detector in the detector array. Bright field detectionis useful for detecting parameters such as reflectivity, contrast, andfeatures such as areal shallow defects.

The bright-field channels may yield useful information on large defectsthat can be discerned by detecting the reflectance at various spots onthe surface of wafer 28. If bright-field inspection at the proposedresolution is found to be useful, then the appropriate fiber channelscan be set up in exactly the same manner as those in the case of thedark-field channels, where a detector array in addition to array 36 isemployed for bright field detection. Bright-field and dark-fieldradiation could also be detected sequentially using the sameelectronics. Alternatively, they may be used simultaneously usingseparate electronics.

Wafer Scanning

Wafer 28 is supported on a chuck (not shown) which is rotated by meansof a motor 72 and translated in a direction by gear 74 so that theilluminated spots 42 are caused to move and trace a spiral swath on thesurface of wafer 28 to inspect the entire surface of the wafer. Bothvacuum handling and edge handling of the samples are possible. Motor 72and gear 74 are controlled by controller 76 in a manner known to thoseskilled in the art. Thus, in the preferred embodiment, the optical head60 remains stationary and the scanning of the beams 24 across thesurface of the wafer 28 is accomplished by using motor 72, gear 74 andcontroller 76 to move the wafer so that the entire surface of the waferis scanned. Alternatively, the optical head 60 may be caused to move ina manner known to those skilled in the art to trace the spiral path oranother type of scan path for scanning wafer 28. The wafer may also bescanned along substantially linear zigzag paths using XY stages.

As noted above, the detector in array 36 may be a photodiode such as anavalanche photodiode; alternatively, it may be a photomultiplier tube.The output of each detector in the detector array 36 is supplied toprocessing circuit 82 where the circuit may comprise a microprocessor,hardware logic or programmable logic circuits, such as those usingFPGA's or dynamic logic or various combinations of these elements.Circuit 82 may be a part of or connected to a computer 84 that is incommunication with controller 76, so that scattered radiation from aparticular detector in array 36 can be matched with a location on thesurface of the wafer 28. Where processing circuit 82 is amicroprocessor, it can be a co-processor within computer 84. Processingcircuit 82 performs certain initial processing of the signals, such assignal amplification and conversion from analog-to-digital form. It maypass the digital signals to computer 84 to perform further processingsuch as die-to-die comparison, or it may perform some or all suchfurther processing itself.

One aspect of the design in system 20 of FIG. 1 is that it is based on astationary optics as described above, and spinning of the wafer asdescribed above, in a manner similar to that in the SP1™ tool, alsoavailable from KLA-Tencor Corporation of San Jose, Calif. It ispreferable for system 20 to have a rather precise spinning action. Forexample, the spinner is capable of some +/−15 microns stability inheight, and uniform spinning on the micron scale. This performance canbe achieved by means of an air bearing stage, for example. By scanningthe wafer surface with multiple spots simultaneously, the scanning ofthe entire wafer surface can be performed in shorter time than if asingle spot were used.

An important consideration that pertains to this spiral scanning actionis that it begins to deviate from very closely linear motion as theposition of the beams approaches the center of the wafer. However, itshould also be remembered that at any given time, one has a precise(within a pixel) knowledge of the position of any of the beams, whichallows one to correct for such scan deviations.

Filters for Reducing Diffraction from Manhattan Geometry and fromPattern

At any given position on the wafer 28 during the beam scanning andinspection process, each of the spots 42 illuminates a number of shapes,which primarily lie along the Manhattan geometry. These shapes allgenerate a two dimensional Sinc function, but with different phases,giving rise to a “+” speckle pattern. As the wafer rotates, this patternalso rotates. If one were to detect all the available scatteredradiation from the wafer, one would also receive parts of thisdiffraction pattern. In the ensuing die-to-die comparison, the presenceof this large background would possibly result in significant errors.

In rectilinear scans, one could resolve these problems by means of astationary spatial filter to filter out the speckle pattern and placingthe detectors along the 45 degrees lines with respect to thehorizontal-vertical directions. In system 20, the rotation of the wafer28 results in a rotating diffraction pattern. This pattern can beeliminated or reduced by placing a “+” shaped filter 90 (i.e. a filterhaving an aperture that passes radiation except for a “+” shaped area),shown more clearly in FIG. 5, directly above the illuminated field, inthe path of the radiation after emergence from the reflective objective32. This filter 90 is made to rotate by means of a motor 89 in FIG. 1under the control of computer 84 in unison with respect to the rotationof wafer 28 under the control of controller 76, resulting in acontinuous obstruction of the diffracted light. Possible approaches tothis issue include the use of ball-bearing based systems, which can bemounted directly at the exit port of the reflecting objective. The useof a programmable liquid crystal filter having an aperture that ischanged in synchronism with the rotational motion of the sample surfaceunder the control of computer 84 to implement filter 90 instead of amechanically rotated one as described above may be viable for lowrotation rates of the wafer.

In addition to diffraction from the Manhattan Geometry, the presence ofany periodic structures such as arrays in DRAMs on the surface of thewafer may also give rise to a two-dimensional Fourier components whenilluminated with normal incidence radiation. If the directions of theexpected pattern scatter from the surface are known, spatial filters maybe designed to block such scattering, thereby detecting only the scatterby anomalies on the surface. FIG. 6 is a schematic view illustrating thetwo-dimensional Fourier components of an array structure that isperiodic in the X and Y directions when illuminated with normalincidence radiation. As the sample rotates, all of the spots at theintersections of the X-Y lines will rotate, thereby generating circles91. These circles represent the loci of the Fourier components as thewafer is rotated. The dark opaque circle at the center is the blockageof the collection space caused by stop 62 in FIG. 1.

From FIG. 6, it is noted that there are gaps in between the circleswhere there are no Fourier components. At least in theory, it ispossible to construct a programmable filter (e.g. a liquid crystalfilter) in which annular bands of arbitrary radii are blocked out. Asimple spatial filter may be constructed also to achieve many of theobjectives herein. Thus, if the cell size of a regular memory array onthe wafer is such that its X and Y dimensions are not larger than about3.5 microns, for example, this means that for 488 nanometers wavelengthradiation used in the illumination beams 24, the first Fourier componentis at about 8° to the normal direction 36. Therefore, if a spatialfilter such as 92 of FIG. 1 is employed, blocking all collectedradiation in the narrow channel that is at 8° or more to the normaldirection 36 will leave an annular gap of 2 or 3° ranging from the rimof the central obscuration (i.e. 5 or 6°) to the rim of the variableaperture at about 8°. Under these conditions, as the wafer spins, noFourier components can possibly get through the annual gap and scatterfrom the array is suppressed. In one embodiment, the spatial filter 92in FIG. 1 used leaves an annular gap between about 5 to 90 from thenormal direction 64 to the surface 28 of the wafer in FIG. 1. For DRAMstructures of smaller cell sizes, such annular aperture type spatialfilter may not even be necessary. While both filters 90 and 92 areemployed in the embodiment of FIG. 1, it will be understood that forcertain applications, the use of only one of the two filters may beadequate and is within the scope of the invention.

It will be noted that even though the collection objective 32 focusesradiation scattered from an array of spots 42, such scattered radiationfrom the spots are focused towards the optical fiber array 34 through asmall area at the focal plane of the objective, so that by placingfilter 90 and/or filter 92 at or close to the focal plane, theabove-described effects can be achieved with respect to the scatteredradiation from all of the illuminated spots 42 in the array of spots.

Detection Channels

Individual APD's, or one or more arrays of APD's, may be used asdetectors in array 36 for each of the dark-field channels. Thesedetectors provide close to photon noise limited performance for typicalillumination levels. Pin diodes, or one or more arrays of pin-diodes maybe used If bright-field channels are considered important, then aseparate APD board may be provided for those, or an array of PIN diodes.

In the preferred embodiment, each APD channel has its own settablesupply, preamplifier, and analog-to-digital converter (ADC), which canbe operated at up to 60 MHz. That is, the potential of this system interms of data rate approaches 5-10 GHz, even though a practical datarate may be somewhat lower. It is important to note that the detectionelectronics part of the design in this case may be completely separatefrom the front-end optics, such as the optical head 60, whichnecessarily results in a compact optical head. The optical head may bereadily integrated into semiconductor processing equipment 88, so thatit is more convenient for anomalies on the wafer surface to be detectedduring processing or between processing steps by means of semiconductorprocessing equipment 88.

Processing Circuit 82

Preferably the detected signals are directed into a massive bank ofrandom access memory (RAM) in circuit 82, capable of holding up to 85Gbytes of data. As the wafer is scanned the data are gathered from thevarious dice at different locations on the wafer. Subsequent imageprocessing is primarily based on a die-to-die comparison process,applied to side-by-side dice, in a rectilinear direction, much in thesame way as that in conventional systems, such as the AIT™ systemsavailable from KLA-Tencor Corporation of San Jose, Calif.

Because of the fact that the scanning is performed in a spiral ratherthan rectilinear fashion, the die-die comparison may be performed on astored version of the 12-bit gray scale data. To achieve this, it willnot be necessary to store the data from the entire wafer, rather only asufficient amount to enable die-die on the present location.Nevertheless, for some applications, it may be desirable to providesufficient memory to store an entire wafer map. At a pixel size of1.25×1.25 microns, a 300 mm wafer has approximately 45 gigapixels. Tostore all pixels as 12-bit values, some 70 GB of memory is preferred.The processing power required must be sufficient to keep up with thepixel rate. A typical pixel rate for some embodiments can be about 1Gpixels/sec. Higher speeds are also possible.

In one embodiment where the scanning is non-rectilinear, it may not bepossible simply to retain the image data for a single swath in order todo die-to-die comparison, as the AIT and other rectilinear scanningdie-to-die machines do. However, by retaining all pixel information onmore than one swath revolution of data on the wafer as it comes in, andby concurrently comparing incoming pixels with those of a reference diewhich is chosen so that its pixels are acquired a little sooner, eachdie can be compared with a reference die during the wafer scan; when thespiral scan is complete, the processing will be nearly finished.

As noted above, a reference die may be chosen so that its pixels areacquired a little sooner, so that each die can be compared with areference die during the wafer scan. This is illustrated in FIG. 7 whichis a schematic illustration of data obtained from an annular region 94of the wafer. The wafer may be scanned beginning at a point on or nearthe circumference of the wafer, or at or near the center of the wafer.Assuming that the spiral path scan of the array of spots 42 starts outat the circumference of the wafer and spirals in towards the center ofthe wafer during the scanning, a reference die 96 may be defined at orclose to the outer circumference of the annular region 94. Therefore,when the data from the target die 98 is obtained, such data may becompared to the data in the reference die obtained earlier for anomalydetection. Obviously, die-to-die comparison using dice data acquiredearlier from a reference die different from die 96 may be used insteadand is within the scope of the invention.

FIG. 8 is a schematic diagram of an optical inspection and imagingsystem illustrating an alternative embodiment of the invention. Insteadof using two separate objectives, one for illumination and the other forcollection, the embodiment 100 of FIG. 8 employs a single objective forthis purpose, although different portions of the objective 102 may beemployed for illumination and for collection. Thus, as shown in FIG. 8,a laser beam from laser 22 is reflected by a mirror or beamsplitter 66and is diffracted into an array of beams 24 by means of diffractiveelement 26 a. Beams 24 are reflected by a center reflective portion 104a of beamsplitter 104 to lens 102. In the preferred embodiment, beams 24are focused by a center portion 102 a of the aperture of lens 102 to thesurface of wafer 28. Scattered radiation from the illuminated spots 42are collected by lens 102 and directed towards the beamsplitter 104. Thecenter reflective portion 104 a acts as an aperture stop that preventsspecular reflection from the surface of the wafer from reaching theoptical fiber array 34. Thus, only the scattered radiation collected bythe circumferential portion 102 b of the aperture of lens 102 can passthrough the beamsplitter 104 and focused by lens 38 towards the detectoror optical fiber array 34. Preferably, the portion 102 b for collectingscattered radiation is larger than the portion 102 a used forillumination, which enhances the sensitivity of detection.

To simplify FIG. 8, the components shown in FIG. 1 for moving the wafer,the bright field channels, the processing circuit and computer have beenomitted from the figure. The embodiment of FIG. 8 has the advantage thatit is even more compact compared to the embodiment of FIG. 1, since asingle objective is used for both illumination and collection. Insteadof using a lens as shown in FIG. 8, it may also be possible to use areflecting objective to ensure easy operation and a large wavelengthrange. Instead of using a center portion 102 a for focusing theillumination beams and a circumferential portion 102 b for collectingthe scattered radiation, the arrangement in FIG. 8 can also be modifiedby directing the illumination beams 24 through a side portion of theobjective, such as the left side of the objective 102 and using theother side, such as the right side, for collection of the scatteredradiation. Such and other variations are within the scope of theinvention. It will be noted that where the paths of illumination beamsare at oblique angles to the surface of sample 28, at least onedimension of the illuminated spots may be such that it is not less thanabout 5 microns. Similar considerations to those described above alsoapply to the bright-field channels in the embodiment of FIG. 8 (notshown in FIG. 8) similar to channels 70 of FIG. 1. However, in this casethe signal generated in each bright-field channel is due to the lightreflected off each of the spots.

In the embodiment of FIG. 1 described above, the illumination beams 24are directed towards the wafer surface in directions that aresubstantially normal to the surface of the wafer. This is not required,however. Thus, for some applications, the illumination beams may bedirected towards a wafer surface at an oblique angle such as along thepaths 24 a indicated by the dotted line in FIG. 1, so that at least onedimension of the illuminated spots is not less than about 5 microns.Thus, especially for the inspection of unpatterned surfaces such asunpatterned wafers, illuminating the wafer surface at an oblique anglemay be desirable for some applications. The same applies to theembodiment of FIG. 8.

FIG. 9 is a top schematic view of an optical inspection and imagingsystem to illustrate yet another embodiment of the invention. As shownin FIG. 9, a single line of illumination beams is supplied at an obliqueangle along direction 202 to the surface of a wafer 28, only a portionof which is shown in FIG. 9. The single line of illumination beams (notshown) illuminate a single file of elongated illuminated spots 204 onthe surface of the wafer. Preferably, the beamsplitter (not shown inFIG. 9) that is used to generate the single file of illumination beamsis oriented at or near an angle, such as 45° for example, relative tothe plane of incidence of the illumination beams, so that the line 204 aconnecting the centers of the spots 204 is also at or near 45° withrespect to the plane of incidence. In this context, the plane ofincidence is defined by a plane containing the illumination direction202 and a line 203 (pointing out of the plane of the paper) intersectingdirection 202 and normal to the surface of the wafer 28. Thus, ifdirection 202 is regarded as an axis of a coordinate system, the line204 a connecting the centers of the spots 204 is substantially at +45°to such axis.

Radiation scattered from the spots 204 are collected along directionssubstantially perpendicular to line 204 a by objectives 210 and 212located above the plane of the surface inspected and on opposite sidesof line 204 a. Objective 210 images the scattered radiation from eachspot 204 onto a corresponding forward channel or detector in the opticalfiber array 34′ or detector array 36′. Similarly, objective 212 imagesthe scattered radiation from each spot 204 to its correspondingbackscatter fiber or detector in the fiberoptic array 34″ or detectorarray 36″. It will be noted that objectives 210 and 212 may be situatedso that all of the spots 204 are substantially within their focalplanes. As shown in FIG. 9, objective 212 will collect the forwardscattered radiation and objective 210 will collect the back scatteredradiation. Instead of using lenses as shown in FIG. 9, objectives 210and 212 may also be reflective objectives.

Alternatively, the beamsplitter that is used to generate the single fileof illumination beams may be oriented at −45° with respect to the planeof incidence so that the spots (not shown in FIG. 9) would form a singlefile oriented at −45° to direction 202, and line 204 b connecting thecenters of the spots at such new locations is also at substantially −45°with respect to the plane of incidence.

If the beamsplitter for generating the array of illumination beams areoriented at −45° with respect to the plane of incidence, then thecollection objectives 210 and 212 would also need to be rotated by 90°so that the spots 204 arranged with their centers along the line 204 bwould again be within their focal planes and that these objectives wouldagain collect radiation scattered in directions substantiallyperpendicular to the line 204 b.

Instead of collecting and imaging scattered radiation in directionsperpendicular to the line joining the centers of the illuminated spots204 as described above, it is also possible to collect and image thescatter radiation in a double dark field configuration. In suchconfiguration, the two objectives would be at locations indicated indotted lines 210′ and 212′ where scattered radiation is collectedsubstantially at +90 and −90 degrees azimuthal angle relative to theillumination beams as they reach the surface; the fiber channels ordetectors have been omitted in such configuration to simplify thefigure. In a double dark field configuration, different spots along theline 204 a or 204 b will be located at different distances from theobjectives so that at least some of them will be out of focus. Eventhough some of the spots 204 will be out of focus or somewhat out offocus, this may not have significant adverse effects on someapplications, such as unpatterned surface inspection. Obviously only oneof the two objectives 210 and 212 (or 210′ and 212′) may suffice forsome applications, so that one of them can be omitted. It is alsopossible to place collection optics 210″ and a detector array 36 or acollection of individual detectors (not shown) directly above the areaof the surface of sample (and therefore in the plane of incidence ofbeams along direction 202) inspected in a single dark fieldconfiguration to detect surface anomalies, such as in the configurationshown in FIG. 10. When the collection optics 210″ is in such position,it images to the detector array or detector collection the scatteredradiation in at least one direction that is substantially normal to thesurface. Preferably the collection optics 210″ used has a largenumerical aperture for increased sensitivity.

If the illumination beams are polarized, it may be desirable to insert apolarizer between each of the two objectives 210 and 210′ and theircorresponding fiber or detection channels. Thus, in the presence of adielectric material such as silicon oxide, circularly polarizedradiation in the illumination beam may be preferable. The presence ofsmall defects may cause P-polarized radiation to be more efficientlyscattered. If S-polarized radiation is employed in the illuminationbeams, scattering caused by the presence of roughness on the surface canbe further suppressed if only S-polarized light is collected. For thispurpose polarizers may be placed in the paths of beams 24 and polarizers220 and 222 may be placed in the collection path for enabling thedetection of polarized radiation components, which may in turn indicatethe type of anomalies present on the wafer. Corresponding polarizers maybe placed along the collection paths in the double dark fieldembodiments. Instead of using refractive objectives such as lenses 210,210′, 212, 212′, reflective objectives may be used which can be used forcollection over a large wavelength range.

While the invention has been described above by reference to variousembodiments, it will be understood that changes and modifications may bemade without departing from the scope of the invention, which is to bedefined only by the appended claims and their equivalents. For example,while the embodiments are illustrated with respect to wafer anomalydetection, the invention may be used for anomaly detection on othertypes of surfaces as well, such as flat panel displays, magnetic andoptical heads, disks and so on. All of the references referred to aboveare incorporated herein by reference in their entirety.

1. A method for detecting anomalies of a surface, comprising: focusingillumination beams of radiation to an array of spots on the surface;imaging radiation reflected from said spots onto a first array ofreceivers or detectors so that each receiver in the first array receivesradiation from a corresponding spot in the array of spots; and imagingscattered radiation from said spots onto a second array of receivers ordetectors in a dark field imaging scheme so that each receiver ordetector in the second array receives radiation from a correspondingspot.
 2. The method of claim 1, wherein said scattered radiation fromsaid spots is imaged in the dark field imaging scheme by means ofreflective optics.
 3. The method of claim 1, further comprisingselecting a wavelength and supplying the illumination beams of radiationso that the radiation comprises a component of the selected wavelengthin a UV, deep UV, visible or infrared wavelength range, said supplyingcomprising passing a beam of radiation of the selected wavelengthcomponent through a diffracting element to form the illumination beams.4. The method of claim 3, further comprising altering the selectedwavelength of the wavelength component of the illumination beams focusedin the focusing, and replacing the diffracting element by anotherdiffracting element so that spot separation of the said spots remainsubstantially unchanged by the altering
 5. The method of claim 1,wherein the focusing focuses the beams to a one or two dimensional arrayof spots, said method further comprising causing rotational motionbetween the surface and the beams so that the spots scan overoverlapping paths.
 6. The method of claim 5, wherein the causing causesrotational motion of the surface while leaving the beams atsubstantially stationary positions.
 7. The method of claim 1, whereinthe focusing focuses the beams to a two dimensional array of spots of apredetermined spot size, and so that adjacent spots are spaced apart bya spacing such that the overlapping paths of adjacent spots overlap byabout ⅔ or ¼ of the predetermined spot size.
 8. The method of claim 1,said surface comprising a surface of an unpatterned semiconductor wafer,wherein the focusing comprises focusing the beams to the surface indirections that are oblique to the surface and so that at least onedimension of the spots is not less than about 5 microns.
 9. An apparatusfor detecting anomalies of a surface, comprising: illumination opticsfocusing illumination beams of radiation to an array of spots on thesurface; bright field imaging optics imaging radiation reflected fromsaid spots onto a first array of receivers or detectors so that eachreceiver in the first array receives radiation from a corresponding spotin the array of spots; and dark field imaging optics imaging scatteredradiation from said spots onto a second array of receivers or detectorsso that each receiver or detector in the second array receives radiationfrom a corresponding spot.
 10. The apparatus of claim 9, furthercomprising means for supplying a beam of radiation of a selectedwavelength in a UV, deep UV, visible or infrared wavelength range, and adiffracting element that diffracts the beam of radiation of the selectedwavelength component to form the illumination beams.
 11. The apparatusof claim 10, said supplying means comprising an optical source thatsupplies radiation of a wavelength selectable from a plurality ofwavelengths, said apparatus comprising a plurality of diffractingelements, each element designed to diffract radiation at one of theplurality of wavelengths so that spot separation of the spots remainssubstantially unchanged when the source selects and supplies radiationsubstantially at a different one of the plurality of wavelengths thanpreviously.
 12. The apparatus of claim 9, said illumination opticscomprising a first objective, said imaging optics comprising a secondobjective having a numerical aperture that is larger than that of thefirst objective.
 13. The apparatus of claim 12, wherein said dark fieldimaging optics images scattered radiation from said spots onto thesecond array of receivers or detectors without employing the firstobjective.
 14. The apparatus of claim 9, wherein the illumination opticsfocuses the beams to a one or two dimensional array of spots, saidapparatus further comprising an instrument causing rotational motionbetween the surface and the beams so that the spots scan overoverlapping paths, wherein the instrument causes rotational motion ofthe surface while leaving the beams at substantially stationarypositions.
 15. The apparatus of claim 14, wherein the dark field imagingoptics is substantially rotationally symmetric about the rotationalaxis.
 16. The apparatus of claim 9, wherein the illumination opticsfocuses the beams to a two dimensional array of spots of a predeterminedspot size, and so that adjacent spots are spaced apart by a spacing suchthat the overlapping paths of adjacent spots overlap by about ⅔ or ¾ ofthe predetermined spot size.
 17. The apparatus of claim 9, wherein theillumination optics focuses the beams to substantially circular spots onthe surface.
 18. The apparatus of claim 9, wherein said dark fieldimaging optics comprises a second objective having an axis in adirection at or near a normal direction to the surface.
 19. Theapparatus of claim 9, said illumination optics comprising an objectivewith a numerical aperture not more than about 0.8.
 20. The apparatus ofclaim 9, said illumination optics comprising a reflective surfacereflecting the beams to the surface, said reflective surface located ina collection path of the dark field imaging optics to block specularreflections of the beams from the surface from reaching the second arrayof receivers or detectors.